Pixel sensor cells (e.g., CMOS imagers) can suffer from added noise and reduced dynamic range due to reintroduction of stored FET-channel charge into the photo-collection region following an erase. For example, after erasing a photocell of charge, the pass gate is switched off, prior to exposure to a new image. In switching the pass gate off, some channel charge leaks back into the photo collector, contaminating the erased state and raising the noise floor (black level).
To address this issue, pixel sensor cells have been manufactured using doped polysilicon gate-electrodes with work function which changes laterally across the transfer gate. However, with doped polysilicon gates this is not scalable due to the length-scale of dopant interdiffusion in the polysilicon. Also, graded gate work-function has been used to provide a lateral channel field to help empty this pass-gate charge away from the photo collector to minimize this effect. However, this adds material complexities to the fabrication process.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.